hour,day,month,minute

That seems a little counterintuitive, don't you think? Wouldn't it be better to specify the date in the form minute,hour,day,month (or reverse)?

If your script is designed to be run periodically you should check for datetimes smaller or equal than the current datetime and execute the action for all instances you find. You'll need a way to mark that an action has already occurred though, because otherwise you'll just repeat them every time your script runs.

You may also want to look at the "at" utility(man 1 at), that should do most of what you're trying to do here.